I used the quotient rule (I think you did too):
where u is the numerator and v is the denominator. In this case:
So I did it like this:
So you're right!
I think it's a typo or just this in another form.
If you put x=1 into both the equations you get two different answers so they're not the same.
The only way to get as the denominator is to differentiate again. I thought this would be f''(x).
However, I began to differentiate again (using the quotient rule, it's similar to before) and I got a cubic appearing on top (hence this can't be f''(x)).
Try and differentiate f'(x) again and see what you get. I'm pretty sure i'm doing it correctly.